There are four valves in the heart that serve to direct blood flow through the two sides of the heart. On the left (systemic) side of the heart are: (1) the mitral valve, located between the left atrium and the left ventricle, and (2) the aortic valve, located between the left ventricle and the aorta. These two heart valves direct oxygenated blood from the lungs through the left side of the heart and into the aorta for distribution to the body. On the right (pulmonary) side of the heart are: (1) the tricuspid valve, located between the right atrium and the right ventricle, and (2) the pulmonary valve, located between the night ventricle and the pulmonary artery. These two heart valves direct de-oxygenated blood from the body through the right side of the heart and into the pulmonary artery for distribution to the lungs, where the blood becomes re-oxygenated in order to begin the circuit anew.
All four of these heart valves are passive structures in that they do not themselves expend any energy and do not perform any active contractile function. They consist of moveable “leaflets” that open and close in response to differential pressures on either side of the valve. The mitral and tricuspid valves are referred to as “atrioventricular valves” because they are situated between an atrium and ventricle on each side of the heart. The mitral valve has two leaflets and the tricuspid valve has three. The aortic and pulmonary valves are referred to as “semilunar valves” because of the unique appearance of their leaflets, which are shaped somewhat like a half-moon and are more aptly termed “cusps”. The aortic and pulmonary valves each have three cusps.
Heart valves may exhibit abnormal anatomy and function as a result of congenital or acquired valve disease. Congenital valve abnormalities may be well-tolerated for many years only to develop a life-threatening problem in an elderly patient, or may be so severe that emergency surgery is required within the first few hours of life. Acquired valve disease may result from causes such as rheumatic fever, degenerative disorders of the valve tissue, bacterial or fungal infections, and trauma.
Since heart valves are passive structures that simply open and close in response to differential pressures on either side of the particular valve, the problems that can develop with heart valves can be classified into two categories: (1) stenosis, in which a valve does not open properly, and (2) insufficiency (also called regurgitation), in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve or in different valves. Both of these abnormalities increase the workload placed on the heart. The severity of this increased stress on the heart and the patient, and the heart's ability to adapt to it, determine whether the abnormal valve will have to be surgically replaced (or, in some cases, repaired).
Valve repair and valve replacement surgery is described and illustrated in numerous books and articles, and a number of options, including artificial mechanical valves and artificial tissue valves, are currently available. However, the currently available options cannot duplicate the advantages of native (natural) heart valves. Some of the available mechanical valves tend to be very durable, but are problematic in that they are thrombogenic and exhibit relatively poor hemodynamic properties. Some of the available artificial tissue valves may have relatively low thrombogenicity, but lack durability. Additionally, even these artificial tissue valves often do not exhibit hemodynamic properties that approach the advantageous hemodynamic performance of a native valve. Some artificial tissue valves attempt to copy the form of native heart valves. Such artificial tissue valves still fall short in durability and in hemodynamic performance.
Moreover, external valve conduits for right ventricular outflow tract (RVOT) have markedly extended the range of complex congenital heart defects that can be repaired: pulmonary atresia, complex tetralogy of Fallot, double outlet right or left ventricle with pulmonary stenosis, truncus arteriosus, transposition of great arteries with ventricular septal defect and pulmonary stenosis, and other complex forms of congenital heart disease.
Although the immediate and long-term survival of patients is gratifying, all conduits deteriorate, requiring the need to have close, constant follow-up. A large proportion, if not all, will have to be replaced at least once during the patient's lifetime. In the case of children, multiple surgeries are often required, which put the child at risk and can be physically and emotionally draining for the child and his or her family. The development and use of improved substitutes for RVOT conduits could reduce the need for multiple surgeries for children with various congenital heart diseases. Tissue-engineered RVOT conduits could significantly improve the quality of life for such children while decreasing the associated medical costs.
Therefore, a heretofore unaddressed need still exists in the art to address the aforementioned deficiencies and inadequacies.